# Absorption coefficient of air

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 September 12, 2016, 16:16 Absorption coefficient of air #1 New Member   Bhupesh Join Date: Jun 2012 Location: Germany Posts: 14 Rep Power: 13 Hi all, I am quite new to radiation simulation and I am using ANSYS for my problem. My question is about the absorption coefficient Rosseland approximation. I am doing a simulation for heating glass with air in a room. While using the rosseland or DO approximation for radiation, as I have seen in some of other threads that they say to use a very small value of absorption coefficient of air, but if we see the rosseland approximation for radiative conductivity, this will result in a very high value for the same, which would mean that the heat conductivity of air is very high, which is not correct physically. So, I have 2 questions here: 1) Can the absorption coefficient be greater than 1? 2) What I think is that the absorption coefficient of air should have a very high value, such that the rosseland approximation gives small value of thermal conductivity, is this correct? NOTE: I know I should use the DO model for semi-transparent material, but here I am more concerned about the absorption coefficient of air as it can't be taken as zero. Thanks in advance!!

 September 12, 2016, 18:05 #2 Super Moderator     Alex Join Date: Jun 2012 Location: Germany Posts: 3,399 Rep Power: 46 It might be that you are confusing the terms "thermal conductivity" and "radiative thermal conductivity". While air as a gas has a rather low thermal conductivity, its radiative thermal conductivity is indeed rather high. Absorption coefficients can not be greater than 1. This would be in conflict with thermodynamics.

September 13, 2016, 02:32
#3
New Member

Bhupesh
Join Date: Jun 2012
Location: Germany
Posts: 14
Rep Power: 13
Quote:
 Originally Posted by flotus1 It might be that you are confusing the terms "thermal conductivity" and "radiative thermal conductivity". While air as a gas has a rather low thermal conductivity, its radiative thermal conductivity is indeed rather high. Absorption coefficients can not be greater than 1. This would be in conflict with thermodynamics.
I am also a bit confused about "Absorbance" and "Absorption coefficient". I can understand that Absorbance cannot be greater than 1, but I am unable to understand why this should be true for Absorption Coefficient.
Also, if the radiative thermal conductivity of air is high, then taking into account Rosseland model, the total thermal conductivity of air will be very high which doesn't seem to give correct result as in my simulation, the whole room is heated up to very high temperature in a few second if I use very small value of absorption coefficient of air.

 September 13, 2016, 08:40 #4 Super Moderator     Alex Join Date: Jun 2012 Location: Germany Posts: 3,399 Rep Power: 46 I am not too familiar with this model. How does the radiative absorption coefficient alter the thermal conductivity? Formula? Link to the manual of the software you are using?

September 13, 2016, 09:13
#5
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Bhupesh
Join Date: Jun 2012
Location: Germany
Posts: 14
Rep Power: 13
Quote:
 Originally Posted by flotus1 I am not too familiar with this model. How does the radiative absorption coefficient alter the thermal conductivity? Formula? Link to the manual of the software you are using?
I am using ANSYS Fluent for my simulation.
The formula and description of model (Rosseland) can be found here:
http://www.afs.enea.it/project/neptu...th/node113.htm
The definition of term can be found here:
http://www.afs.enea.it/project/neptu...th/node112.htm

 September 13, 2016, 11:11 #6 Super Moderator     Alex Join Date: Jun 2012 Location: Germany Posts: 3,399 Rep Power: 46 While the model apparently uses conductive heat transfer as an analogy to model radiative heat transfer, it clearly distinguishes between radiative and conductive heat flux and radiative conductivity and thermal conductivity respectively. Please ignore what I said about the absorption coefficient, I got the terms mixed up myself. Maybe somebody else can help.

 September 13, 2016, 19:42 #7 New Member   Bhupesh Join Date: Jun 2012 Location: Germany Posts: 14 Rep Power: 13 @flotus1, thanks for your time..